Effect of Reaction Temperature on the Performance of Thermal Swing Sorption-Enhanced Reaction Process for Simultaneous Production of Fuel-Cell-Grade H2 and Compressed CO2 from Synthesis Gas

Abstract

A novel cyclic thermal swing sorption-enhanced reaction (TSSER) process concept was recently proposed for the simultaneous production of fuel-cell-grade H2 and compressed CO2 from synthesis gas containing CO and H2O. The process carried out the catalytic water−gas shift (WGS) reaction (CO + H2O ↔ CO2 + H2) with simultaneous removal of CO2 from the reaction zone by a reversible, water-tolerant, CO2-selective chemisorbent in order to circumvent the thermodynamic limitation of the WGS reaction and enhance the rate of the forward reaction. The chemisorbent was periodically regenerated using the principles of thermal swing adsorption by purging the sorber−reactor with superheated steam at different pressures and temperatures. Several intermediate process steps were employed to produce a pure and compressed CO2 byproduct during the thermal desorption process. The present work reports (a) new experimental data demonstrating the concept of the sorption-enhanced WGS reaction at different temperatures using a commercial WGS catalyst and Na2O-promoted alumina as the CO2 chemisorbent and (b) the effect of the sorption−reaction temperature on the TSSER process performance estimated by model simulation. Relatively slower kinetics of the sorption-enhanced WGS reaction imposes a lower bound (∼200 °C), whereas the thermal stability of the chemisorbent and the use of carbon steel sorber−reactors set the upper bound (∼550 °C) of temperatures for practical operation of the TSSER process. Simulated process performances (sorption−reaction at 200 and 400 °C and regeneration at 550 °C) show that the operation of the sorption−reaction step at 200 °C increases the H2 and CO2 productivities of the process by ∼38% and 35%, respectively, without changing (a) the number of moles of H2 produced per mole of CO in the feed gas or (b) the net CO2 recovery as a compressed byproduct gas. The total steam duty for the sorbent regeneration increases by ∼14% for operation at the lower sorption−reaction temperature. Another major benefit of operation at the lower reaction temperature is a very large increase in the pressure of the CO2 byproduct (e.g., 40 and 21 atm at 200 and 400 °C, respectively) when the reactor feed gas contained 20% CO + 80% H2O at a total pressure of 15 atm.